Recently, with the rapid proliferation of the Internet, there has been a demand for a further increase in communication traffic. Under the circumstance, the transmission rate per unit channel in a system has increased as well as the number of channels based on wavelength division multiplexing (to be abbreviated as WDM hereinafter). WDM is a scheme which can simultaneously transmit a plurality of optical signals assigned to different carrier wavelengths (channels), and allows an increase in communication capacity in accordance with the number of channels. The respective channel wavelengths are sufficiently separated from each other. If, for example, data are modulated at 10 gigabits/sec per channel to transmit 100-channel data using one common optical fiber, the communication capacity reaches one terabit/sec.
As a wavelength band used for recent medium/long-distance optical communication, the C-band (1,530 to 1,570 nm) which can be amplified by an optical fiber amplifier (erbium-doped fiber amplifier to be abbreviated as an EDFA hereinafter) is widely used. In general, laser devices are prepared for standard channels used for optical communications in accordance with the respective wavelengths. That is, 100 types of laser devices are required for 100 channels. Because of this arises a problem of increasing the cost of inventory management and stocktaking. For the above reason, there is a demand for the commercialization of a wavelength tunable laser device which can cover alone the C-band as a wavelength band which can be amplified by an EDFA in medium/long-distance communication. If the entire C-band can be covered by one laser device, it allows both the manufacturer and the user to handle only a single type of laser device. This makes it possible to greatly reduce the cost of inventory management and stocktaking.
On the other hand, there is also a demand for the construction of a flexible network which allows dynamic path setting in accordance with changes in traffic and troubles. That is, improvements in the infrastructure of networks capable of providing more diversified services are required. A technique of freely controlling wavelengths is indispensable to construct such a large-capacity, high-performance, high-reliability photonic network. A wavelength tunable laser has therefore become a very important system key device.
As a wavelength tunable laser which satisfies such requirements, Japanese Patent Laid-Open No. 2003-023208 (to be referred to as reference 1 hereinafter) has disclosed a structure in which the oscillation wavelengths of a plurality of distributed feedback (to be abbreviated as DFB) lasers arranged in parallel are shifted in advance, coarse wavelength adjustment is performed by switching the lasers, and fine wavelength adjustment is performed by using changes in refractive index with changes in temperature. According to the wavelength tunable laser disclosed in reference 1, output ports must be coupled as one port to an optical fiber, and hence it is necessary to use an optical coupler which couples the output ports of the respective DFB lasers into one output port. If, therefore, the number of DFB lasers arranged in parallel increases, the loss in the optical coupler increases. That is, there is a tradeoff relationship between wavelength tuning range and optical output power.
A wavelength tunable laser based on a DFB laser allows fine adjustment based on temperature, and hence can be used in combination with the wavelength locker disclosed in Japanese Patent Laid-Open No. 2001-257419 (to be referred to as reference 2 hereinafter). A wavelength locker is an etalon-type filter having a periodic transmission amplitude on the frequency axis. Since the transmitted light intensity of the etalon-type filter sensitively changes in accordance with the laser frequency near the amplitude center, it is possible to tune the laser frequency to a desired laser frequency by detecting a transmitted light intensity with a monitor current in a photoelectric conversion element. As described above, a combination of a DFB laser and a wavelength locker is an effective means for accurately locking the laser wavelength to a standard channel wavelength.
As a wavelength tunable laser which is free from the above tradeoff relationship and satisfies the requirement of freely controlling wavelengths, an external cavity wavelength tunable laser is proposed, which forms an external cavity by using a semiconductor optical amplifier and an external reflecting mirror, and implements a wavelength selection characteristic by inserting a wavelength tunable filter, a wavelength tunable mirror, and the like in the external cavity. This external cavity wavelength tunable laser can relatively easily obtain a wavelength tuning range which covers the entire C-band, and hence has been extensively researched and developed.
Most of the basic characteristics of an external cavity wavelength tunable laser are determined by a wavelength tunable filter and a wavelength tunable mirror inserted in a cavity. Therefore, various wavelength tunable filters and wavelength tunable mirrors having excellent characteristics have been developed. As wavelength tunable filters, there are available, for example, a filter designed to rotate an etalon which is disclosed in Japanese Patent Laid-Open No. 4-69987 (to be referred to as reference 3 hereinafter), a filter designed to rotate a diffraction grating which is disclosed in Japanese Patent Laid-Open No. 5-48200 (to be referred to as reference 4 hereinafter), and an acoustooptic filter and dielectric filter disclosed in Japanese Patent Laid-Open No. 2000-261086 (to be referred to as reference 5). As a wavelength tunable mirror, for example, there is available an electrically controlled wavelength tunable mirror with an external mirror itself having a wavelength tunable characteristic which is disclosed in U.S. Pat. No. 6,215,928B1 (to be referred to as reference 6 hereinafter).
There are various types of methods of forming an external cavity wavelength tunable laser by using a wavelength tunable filter or wavelength tunable mirror. The following arrangements are effective in implementing a high-performance light source: an arrangement obtained by combining a gain medium such as a semiconductor optical amplifier, a wavelength selection filter having a periodic transmission characteristic on the frequency axis (to be abbreviated as a wavelength selection filter hereinafter), a wavelength tunable filter, and a reflecting mirror as disclosed in reference 5; and an arrangement obtained by combining a wavelength selection filter and a wavelength tunable mirror as disclosed in “K. Mizutani, et al., “Over 15 dBm Fiber-Coupled Power Broadband External Cavity Tunable Laser using a Voltage-Controlled Tunable Mirror”, ECOC (European Conference on Optical Communication) Proceedings, Vol. 4, Th2.4.5, 2004, pp. 868-869” (to be referred to as reference 7 hereinafter). As a wavelength selection filter, an etalon having a periodic transmission characteristic on the frequency axis is used. As a wavelength tunable filter, an acoustooptic filter is used. As a wavelength tunable mirror, an electrically controlled wavelength tunable mirror is used.
The principle of wavelength selection operation by an external cavity wavelength tunable laser will be briefly described with reference to FIGS. 13, 14A, 14B, 14C, and 14D. FIG. 13 is a side view showing the arrangement of a conventional external cavity wavelength tunable laser device. FIGS. 14A, 14B, 14C, and 14D are views for explaining the laser oscillation modes of the external cavity wavelength tunable laser device in FIG. 13. Referring to FIG. 13, reference numeral 51 denotes a semiconductor element; 52, a semiconductor optical amplifier 52; 53, a low-reflection coated surface; 54, a nonreflective coated surface; 55, collimating lens; 56, etalon; 57, a wavelength tunable filter; 58, a total reflection mirror; 59, a subcarrier; and 101, a temperature controller. The low-reflection coated surface 53, semiconductor optical amplifier 52, nonreflective coated surface 54, collimating lens 55, etalon 56, wavelength tunable filter 57, and total reflection mirror 58 constitute an external cavity. FIG. 14A is a graph showing the transmission characteristic of the wavelength tunable filter 57. FIG. 14B is a graph showing the transmission characteristic of the etalon 56. The 14C is a graph showing the Fabry-Perot modes of the external cavity. FIG. 14D is a graph showing the laser oscillation modes of the external cavity.
Light output from the semiconductor optical amplifier 52 as a gain medium contains many Fabry-Perot modes 63 dependent on the total length of the external cavity, as shown in FIG. 14C. Of these modes, only a plurality of modes which coincide with the period of a periodic transmission band 62 (FIG. 14B) of the etalon 56 as a wavelength selection filter are selected and made to pass through the wavelength selection filter. At this time, since the Fabry-Perot modes which cannot transmit the wavelength selection filter are suppressed, this filter has a merit of easily suppressing sub-modes other than a channel even in an arrangement in which the frequency intervals between Fabry-Perot modes are relatively short, i.e., the total length of the external cavity is relatively large.
The wavelength tunable filter 57 having a transmission characteristic 61 shown in FIG. 14A selects only one of a plurality of modes transmitted through the wavelength selection filter. The selected mode is then transmitted through the wavelength tunable filter 57. Reference numeral 64 in FIG. 14D denotes a mode which is transmitted through the wavelength tunable filter 57. The light transmitted through the wavelength tunable filter 57 is reflected by the total reflection mirror 58, and finally returns to the semiconductor optical amplifier 52. In this manner, a feedback loop is formed. The arrangement in FIG. 13 can relatively easily implement a wavelength tunable laser with high mode stability. In addition, a wavelength selection characteristic can be implemented by relatively simple control.
In the arrangement shown in FIG. 13, the periodic wavelength of the wavelength selection filter is fixed, and the wavelength at a transmission peak coincides with a standard channel for optical communication. According to the arrangement in FIG. 13, since the wavelength selection filter is placed inside the external cavity, a wavelength accuracy can be obtained within the channel accuracy of the wavelength selection filter without using any wavelength locker required in a wavelength tunable DFB laser.
In addition, according to the arrangement in FIG. 13, arranging the filter, the mirror, and the like to make a light beam emitting from the semiconductor optical amplifier linearly travel can miniaturize the external cavity. This facilitates physical placement for achieving a desired cavity mode interval even in an actual implementation step. That is, this can be said to be an excellent arrangement. Such an arrangement is typified by the laser device disclosed in Japanese Patent Laid-Open No. 2004-356504 (to be referred to as reference 8 hereinafter).
However, the external cavity wavelength tunable lasers disclosed in references 5 and 8 also include several problems.
The first problem is that such an arrangement is not suitable for the implementation of a high power laser. This is because, since the wavelength selection filter is inserted between the semiconductor optical amplifier and the total reflection mirror, optical loss occurs when light is transmitted through the wavelength selection filter, resulting in interference with an increase in the power of the laser.
The reason why the conventional external cavity wavelength tunable laser is not suitable for an increase in power will be described in more detail. As described above, an etalon is a typical example of a wavelength selection filter. An etalon generally has a Fabry-Perot structure in which two opposing reflecting mirrors are arranged at a given fixed interval. A simplest example is a glass cube. The interface between the glass and the air functions as a reflecting mirror. The thickness of the glass cube is the fixed interval. Since light is repeatedly reflected between these two reflecting mirrors, optical resonance occurs. In this case, a transmission peak repeatedly appears with respect to the frequency (wavelength) of light, and a period FSR (Free Spectral Range) can be expressed byFSR=C/(2nd)  (1)where C is a light velocity (300,000 km/s). If, for example, the refractive index and thickness of the glass are n=1.5 and d=2 mm, respectively, the period FSR of transmission peaks is just 50 GHz. That is, the period FSR can be made to coincide with the standard channel interval.
As described above, a channel wavelength at which light is completely transmitted through the etalon due to interference periodically exits in the etalon. That is, when light with other wavelengths strike the etalon, the optical power is partly or totally reflected due to interference. In addition, if the reflecting surface of the etalon is perfectly vertical to a light beam, light with the periodic channel wavelength is completely transmitted through the etalon. On the other hand, when an optical power component which is not transmitted through the etalon is reflected by the etalon and returns to the semiconductor optical amplifier, the light interferes with the laser oscillation mode stability. In practice, therefore, the etalon is tilted with respect to a light beam to prevent reflected light from the etalon from returning to the semiconductor optical amplifier.
If, however, the etalon is tilted with respect to a light beam, the light beam inside the etalon causes a positional shift every time it is reflected, resulting in a reduction in interference effect. For this reason, even light with a wavelength which is perfectly transmitted through the etalon is not partly used for interference and emits. This is the reason for optical loss. Loss always occurs in such a wavelength selection filter in addition to an etalon. This causes excessive optical loss for the external cavity, and hence hinders an increase in laser power.
The second problem in the conventional external cavity wavelength tunable laser is that some limitation is imposed on wavelength accuracy obtained with respect to the standard channel wavelength. The reason why some limitation is imposed on wavelength accuracy will be described in detail below. The external cavity wavelength tunable laser using the wavelength selection filter is accompanied by a mechanism of performing phase adjustment for the laser oscillation mode using some method. As disclosed in reference 7, in some case, a phase adjustment mechanism is integrated with a semiconductor optical amplifier. Using the phase adjustment mechanism makes it possible to finely adjust a laser oscillation wavelength. In general, since the transmission peak wavelength of the wavelength selection filter is made to coincide with the standard channel used in optical communication, the laser oscillation wavelength is simply controlled to minimize loss in the wavelength selection filter. That is, in practice, control is performed to maximize the laser oscillation power near the transmission peak wavelength of the wavelength selection filter.
However, since the change amount of optical power is small near the transmission peak of the wavelength selection filter, it is difficult to make the laser oscillation wavelength perfectly coincide with the transmission peak of the wavelength selection filter by the above control operation of maximizing optical power. That is, a certain degree of error is unavoidable. Therefore, in comparison with the structure having the wavelength lock mechanism added to the outside of the laser cavity disclosed in reference 2, some limitation is imposed on wavelength accuracy with respect to the standard channel. In general, errors reach about several GHz. Such an error is worse by one order of magnitude or more than the wavelength accuracy based on the wavelength lock mechanism disclosed in reference 2.
The third problem in the conventional external cavity wavelength tunable laser is that the frequency modulation (FM modulation) efficiency is low. This is because the laser oscillation wavelength is locked to the periodic transmission peak wavelength of the wavelength selection filter. The reason why the FM modulation efficiency is low will be described in detail below. The wavelength selection filter internally has a resonance structure like an etalon. Therefore, light reciprocates most often inside the etalon near the wavelength at which light is transmitted most, and the effective optical path length increases manifolds. For this reason, the behavior of wavelength becomes insensitive to laser phase control, i.e., optical path length adjustment.
In recent optical fiber communication, it is known that optical loss in the optical fiber can be reduced by intentionally FM-modulating a laser oscillation wavelength so as to suppress stimulated Brillouin scattering (SBS) in the optical fiber. If, however, a wavelength selection filter is used, the FM modulation efficiency decreases, and SBS cannot be suppressed, resulting in an increase in loss in the optical fiber. This poses a problem in long-distance communication.
The fourth problem of the conventional external cavity wavelength tunable laser is that continuous wavelength tuning operation cannot be performed. This is because laser oscillation is allowed only near the periodic transmission peak wavelength of the wavelength selection filter. This indicates that as the wavelength channel interval decreases and the channel density becomes high in the future, the laser cannot cope with an increase in channel density.
The fifth problem of the conventional external cavity wavelength tunable laser is that the cost is high. As described above, in consideration of the actual application of the laser to a DWDM optical communication system, it is necessary to make the oscillation wavelength always coincide with the standard channel. For this purpose, a basic component always requires a phase adjustment mechanism, and it is necessary to finely adjust the wavelength by using the phase adjustment mechanism at the time of channel wavelength setting. This means that the optical arrangement of the external cavity wavelength tunable laser and its control circuit arrangement become complicated. This becomes a factor that increases the cost and decreases the wavelength switching speed.
As a laser which partly solves the above problem, there is available the external cavity wavelength tunable laser disclosed in J. Berger, et al., “Widely Tunable, Narrow Optical Bandpass Gaussian Filter Using a Silicon Microactuator”, OFC (Optical Fiber Communication Conference) 2003, VOL. 1, TuN2, 2003, pp. 252-253 (to be referred to as reference 9 hereinafter). FIG. 15 shows the arrangement of this external cavity wavelength tunable laser. The external cavity wavelength tunable laser in FIG. 15 comprises a semiconductor optical amplifier 71, collimating lens 72, variable-angle diffraction grating wavelength tunable filter 73, and variable-angle total reflection mirror 74. As a laser having the same arrangement, an external cavity wavelength tunable laser using a variable-angle micro-etalon wavelength tunable filter is disclosed in Japanese Patent Laid-Open No. 2004-348136 (to be referred to as reference 10 hereinafter). The external cavity wavelength tunable laser disclosed in references 9 and 10 do not use any wavelength selection filter such as an etalon, the first, third, and fourth problems of the first to fifth problems can be solved.